Hot water apparatuses include direct-heating hot water apparatuses and indirect-heating hot water apparatuses. Direct-heating hot water apparatuses are designed to heat water circulating in a water circuit by a heat source, and store the heated water circulating in the water circuit in a tank. Indirect-heating hot water apparatuses are designed to heat water circulating in a primary water circuit by a heat source, then heat water circulating in a secondary water circuit by the heated water circulating in the primary water circuit, and then store the heated water circulating in the secondary water circuit in a tank.
Some indirect-heating hot water apparatuses use a fluid circuit, instead of the primary water circuit, in which a fluid other than water, such as brine, circulates.
Patent Document 1 discloses a method for detecting water circuit clogging caused by scale formation of calcium carbonate or the like, in a direct-heating hot water apparatus.
Patent Document 1 discloses the following methods for clogging detection:    (1) measuring changes in the flow rate of water in a water circuit;    (2) measuring changes in the pressure of water in a water circuit;    (3) measuring changes in the output of a pump in a water circuit; and    (4) measuring changes in heating capability.